Magnetic recording medium

ABSTRACT

A magnetic recording medium having an excellent running property as well as a high durability is provided which comprises a non-magnetic support and a magnetic layer provided thereon in which a ferromagnetic fine powder is dispersed in a binder consisting essentially of a copolymer of vinyl chloride, ethylene, vinyl acetate and maleic anhydride, optionally with a polyisocyanate.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a magnetic recording medium and moreparticularly, it is concerned with an improved magnetic recording mediumhaving excellent properties, in particular, good running property anddurability.

2. Description of the Prior Art

In magnetic recording materials, in particular, audio cassette tapes,which have lately been used for music recording, an excellent frequencycharacteristic and sound reproducing capacity are required. With thespread of car stereo and radio-cassette system, an excellent taperunning property and durability are also required. On the other hand, invideo cassette tapes, recording of very high density has been carriedout by shortening the recording wavelength or by narrowing the trackwidth and to this end, a higher S/N ratio and excellent imagereproducing property are required. Furthermore, with the spread ofportable VTR, there is an increased demand for VTR tapes having adecreased total thickness, i.e. less than 20 μm and a much higherrunning durability than in the prior art.

That is, both the audio tapes and video tapes are strongly required tohave more excellent electromagnetic property, tape running property anddurability than those of the prior art. For the purpose of meeting theserequirements, various binder compositions have hitherto been proposed,but satisfactory binders have not been found yet.

At present, mixtures of copolymers of vinyl chloride/vinyl acetate/othervinyl compounds with urethane rubber resins have mainly been used as abinder composition from the standpoint of the dispersibility offerromagnetic powders and the running property or durability of magneticlayers. The copolymers of vinyl chloride/vinyl acetate/other vinylcompounds include vinyl chloride/vinyl acetate/vinyl alcohol copolymers,vinyl chloride/vinyl acetate/maleic anhydride copolymers and the likeand particularly, the vinyl chloride/vinyl acetate/maleic anhydridecopolymers have often been used because of being excellent indispersibility of ferromagnetic powders.

These prior art copolymers of vinyl chloride/vinyl acetate/maleicanhydride have generally compositions of 80 to 90% of vinyl chloride, 10to 15% of vinyl acetate and 0.5 to 3% of maleic anhydride, which arecommercially sold as a commercial name VMCH by Union Carbide Co., asS'LEC M by Sekisui Kagaku Kogyo KK, or as Denka Vinyl 1000 M by DenkiKogyo KK.

When using the prior art copolymers of vinyl chloride/vinylacetate/maleic anhydride, however, the flexibility of a magnetic layerlacks, resulting in necessity of adding a large quantity of plasticizersand a large quantity of soft resins. In the case of adding a largequantity of plasticizers, problems tend to arise that blooming occursand the durability of magnetic layers lowers, while in the case ofjointly using a large quantity of polyurethane resins, problems tend toarise as to the dispersibility of ferromagnetic powders and the taperunning property at a higher temperature and higher humidity.

Therefore, it is often difficult to obtain a magnetic recording mediumwith a sufficient running property and durability required at present bythe prior art combinations of vinyl chloride/vinyl acetate/maleicanhydride copolymers and polyurethane resins.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved magneticrecording medium whereby the above described disadvantages can beovercome.

It is another object of the present invention to provide a magneticrecording medium having an excellent running property and durability.

It is a further object of the present invention to provide a video tapehaving a higher video output and S/N ratio as well as an excellentelectromagnetic property.

These objects can be attained by a magnetic recording medium comprisinga non-magnetic support and a magnetic layer provided thereon in which aferromagnetic powder is dispersed in a binder consisting mainly of acopolymer of vinyl chloride, ethylene, vinyl acetate and maleicanhydride and optionally with a polyisocyanate.

DETAILED DESCRIPTION OF THE INVENTION

The inventors have made studies on various binder compositions formagnetic recording media and consequently, have found that a magneticrecording medium excellent in electromagnetic property, running propertyand durability can be obtained by using a binder consisting mainly ofvinyl chloride/ethylene/vinyl acetate/maleic anhydride copolymeroptionally with a polyisocyanate. This invention is based on thisfinding.

The vinyl chloride/ethylene/vinyl acetate/maleic anhydride copolymeraccording to the present invention has generally a composition of 50 to95% by weight of vinyl chloride, 0.5 to 20% by weight of ethylene, 0.5to 20% by weight of vinyl acetate and 0.5 to 5% by weight of maleicanhydride, preferably 60 to 90% by weight of vinyl chloride, 1 to 10% byweight of ethylene, 1 to 15% by weight of vinyl acetate and 1 to 3%weight of maleic anhydride. The polymerization degree of the copolymerof the present invention is preferably about 200 to 2000, morepreferably 300 to 1000.

The vinyl chloride/ethylene/vinyl acetate/maleic anhydride copolymer ofthe present invention can be prepared by subjecting all the monomers torandom copolymerization in known manner or by preparing first anethylene/vinyl acetate/maleic anhydride copolymer and then subjecting tograft polymerization with vinyl chloride in known manner.

Instead of the maleic anhydride, other carboxyl group-containingmonomers such as acrylic acid, methacrylic acid and the like can beused, but for the purpose of this invention, maleic anhydride is mostsuitable.

In the case of video tapes used under severer conditions than in thecase of audio tapes, it is preferable to add a polyisocyanate to hardenthe magnetic layer. Useful examples of the polyisocyanate used jointlywith the vinyl chloride/ethylene/vinyl acetate/maleic anhydridecopolymer of the present invention are reaction products of three molesof a diisocyanate such as toluylene diisocyanate, xylene diisocyanate orhyxamethylene diisocyanate with 1 mole of trimethylolpropane, biuretadduct compounds of three moles of toluylene diisocyanate, isocyanurateadduct compounds of three moles of toluylene diisocyanate and 2 moles ofhyxamethylene diisocyanate, polymer compounds of diphenylmethanediisocyanate and isophorone diisocyanate.

These compounds are available as commercial names "Colonate L","Colonate HL", "Colonate 2030", "Millionate MR" and "Millionate MTL"from Nippon Polyurethane KK, as "Desmodur L", "Desmodur N", "DesmodurIL" and "Desmodur HL" from Sumitomo Bayer Urethane KK and as "TakenateD-102", "Takenate D-110 N" and "Takenate D-202" from Takeda YakuhinKogyo KK.

In the case of using jointly a polyisocyanate in the present invention,it is generally added to the binder composition in a proportion of about40% or less by weight, since if more than about 40% by weight, theadhesiveness between a magnetic layer and support deteriorates.

The vinyl chloride/ethylene/vinyl acetate/maleic anhydride copolymeraccording to the present invention can optionally be used jointly withother resins miscible with the vinyl chloride/ethylene/vinylacetate/maleic anhydride copolymer, such for example as cellulose resinssuch as nitrocellulose, vinylidene chloride/acrylonitrile copolymers,vinyl chloride/vinyl acetate copolymers, vinyl chloride/vinylacetate/vinyl alcohol copolymers, vinyl chloride/vinyl acetate/maleicanhydride copolymers, polyurethane resins and polyvinyl butyral resins.These resins are generally used in a proportion of about 90% by weightor less, preferably about 60% by weight or less, more preferably about40% by weight or less to the binder composition.

The binder consisting mainly of a vinyl chloride/ethylene/vinylacetate/maleic anhydride copolymer optionally with a polyisocyanateaccording to the present invention is preferably used in a proportion ofabout 10 to 50% by weight, more preferably 15 to 30% by weight to amagnetic substance.

In the present invention, furthermore, plasticizers, stabilizers andlubricants can jointly be used. Details of these additives are describedin "Vinyl Chloride Resins" published by Nikkan Kogyo Shinbunsha.

Preparation of the magnetic recording medium according to the presentinvention is carried out by coating onto a support a magnetic materialobtained by dispersing the above described binder composition, amagnetic powder and additives with an organic solvent, followed bydrying.

Useful examples of the ferromagnetic powders which can be used in thepresent invention are γ-Fe₂ O₃, Co-doped γ-Fe₂ O₃, Fe₃ O₄, Co-doped Fe₃O₄, Berthollide compounds of γ-Fe₂ O₃ and Fe₃ O₄ (FeOx: 1.33<x<1.50),Co-doped Berthollide compounds of γ-Fe₂ O₃ and Fe₃ O₄ (FeOx: 1.33<x<1.50), CrO₂, Co-Ni-P alloys, Co-Ni-Fe alloys, Co-Ni-Fe-B alloys,Fe-Ni-Zn alloys, Fe-Mn-Zn alloys, Fe-Co-Ni-P alloys and Ni-Co alloys, asdescribed in Japanese Patent Publication Nos. 14090/1969, 18372/1970,22062/1972, 22513/1972, 28466/1971, 38755/1971, 4286/1972, 12422/1972,17284/1972, 18509/1972, 18573/1972, 10307/1964 and 39639/1973, U.S. Pat.Nos. 3,026,215, 3,031,341, 3,100,194, 3,242,005 and 3,389,014, BritishPat. Nos. 752,659, 782,762 and 1,007,323, French Pat. No. 1,107,654 andWest German OLS No. 1,281,334.

The ferromagnetic fine powders have a grain size of preferably about 0.2to 1 μm in length with a length to width ratio of 1:1 to 20:1.

In addition to the above described binder and ferromagnetic powder,other additives such as dispersing agents, lubricant, abrasives,antistatic agents and the like can be added to the magnetic recordinglayer.

Suitable dispersing agents are fatty acids containing about 12 to 18carbon atoms represented by the general formula R₁ COOH wherein R₁ is analkyl group containing about 11 to 17 carbon atoms, for example,caprylic acid, capric acid, lauric acid, myristic acid, palmitic acid,stearic acid, oleic acid, elaidic acid, linolic acid, linolenic acid,stearolic acid and the like; metallic soaps comprising the alkali metal(Li, Na, K, etc.) salts or the alkaline earth metal (Mg, Ca, Ba, etc.)salts of the above described fatty acids; and lecithin, etc. Inaddition, higher alcohols containing about 12 or more carbon atoms andthe sulfates thereof can be used. These dispersing agents are generallyused in a proportion of about 1 to 20 parts by weight per 100 parts byweight of a binder. These dispersing agents are described in JapanesePatent Publication Nos. 28369/1964, 17945/1969 and 15001/1973 and U.S.Pat. Nos. 3,387,993 and 3,470,021.

Suitable lubricants which can be used in the present invention includesilicone oils, carbon black, graphite, carbon black graft polymers,molybdenum disulfide, tungsten disulfide, fatty acid esters producedfrom a monocarboxylic acid containing about 12 to 16 carbon atoms and amonohydric alcohol containing about 3 to 12 carbon atoms, fatty acidesters produced from a monocarboxylic fatty acid containing about 17 ormore carbon atoms and a monohydric alcohol in which the total number ofcarbon atoms ranges from about 21 to 23 and the like. These lubricantsare generally used in a proportion of about 0.2 to 20 parts by weightper 100 parts by weight of a binder. These lubricants are described inJapanese Patent Publication Nos. 18064/1966, 23889/1968, 40461/1971,15621/1972, 18482/1972, 28043/1972, 32001/1972, 5042/1975, 14082/1977,18561/1977, 8804/1977, 49803/1977, 67307/1977, and 70811/1977, U.S. Pat.Nos. 3,470,021, 3,492,253, 3,497,411, 3,523,086, 3,625,760, 3,630,772,3,634,253, 3,642,539, 3,687,725, 3,996,407, 4,007,313, 4,007,314,4,018,967 and 4,018,968 IBM Technical Disclosure Bulletin, Vol. 9, No.7, page 779 (December 1966), and ELECTRONIK, No. 12, page 380 (1961),West Germany.

Typical abrasive agents which can be used in the present inventioninclude fused alumina, silicon carbide, chromium oxide, corundum,diamond, synthetic diamond, garnet, emery (main component: corundum andmagnetite) and the like. These abrasive agents have generally a Mohs'hardness of at least 5 and a mean particle size of 0.05 to 5 microns,preferably 0.1 to 2 microns, and are generally added in a proportion of1 to 10 parts by weight to 100 parts by weight of the magnetic powder.Examples of the abrasives are described in Japanese Patent PublicationNos. 18572/1972, 15003/1973, 15004/1973 (U.S. Pat. No. 3,617,378),39402/1974, and 9401/1975, U.S. Pat. Nos. 3,007,807, 3,041,196,3,293,066, 3,630,910, 3,687,725 and 4,015,042 British Pat. No.1,145,349, and West German Pat. No. 853,211 and 1,001,000.

Antistatic agents which can be used in the present invention includeelectrically conductive powders such as graphite, carbon black andcarbon black graft polymers; natural surface active agents such assaponin; nonionic surface active agents such as alkylene oxide based,glycerol based and glycidol based surface active agents; cationicsurface active agents such as heterocyclic compounds, e.g. higheralkylamines, quaternary ammonium salts, pyridine and the like;phosphoniums, sulfoniums and the like; anionic surface active agentscontaining acid groups such as carboxylic acid groups, sulfonic acidgroups, phosphoric acid groups, sulfate groups, phosphate groups and thelike; amphoteric surface active agents such as sulfates or phosphate ofamino acids, amino sulfonic acids, amino alcohols and the like; etc.

Examples of the surface active agents which can be used as antistaticagents are described in U.S. Pat. Nos. 2,271,623, 2,240,472, 2,288,226,2,676,122, 2,676,924, 2,676,975, 2,691,566, 2,727,860, 2,730,498,2,742,379, 2,739,891, 3,068,101, 3,158,484, 3,201,253, 3,210,191,3,294,540, 3,415,649, 3,441,413, 3,442,654, 3,475,174 and 3,545,974,West German Patent Application (OLS) No. 1,942,665, British Pat. Nos.1,077,317 and 1,198,450, Ryohei Oda et al., "Kaimen Kassei Zai no Goseito so no Oyo (Synthesis of Surface Active Agents and TheirApplications), Maki Shoten, Tokyo (1964), A. M. Schwrats et al.,"Surface Active Agents", Interscience Publications Corp., New York(1958), J. P. Sisley et al., "Encyclopedia of Surface Active Agents",Vol. 2, Chemical Publishing Co., New York (1964), "Kaimen Kassei ZaiBinran (Handbook of Surface Active Agents)", 6th Ed., Sangyo Tosho Co.,Tokyo (Dec. 20, 1966), etc.

These surface active agents can be used individually or in combinationwith each other. These surface active agents are generally used asantistatic agents, but in some cases, they are used for other purposes,for example, for improving dispersibility, magnetic properties andlubricity, or as an auxiliary coating agent.

Formation of a magnetic recording layer is carried out by dissolving ordispersing the above described composition in an organic solvent andthen coating the resulting composition onto a support.

Suitable materials which can be used for this support are variousplastics, for example, polyesters such as polyethylene terephthalate,polyethylene-2,6-naphthalate and the like, polyolefins such aspolypropylene and the like, cellulose derivatives such as cellulosetriacetate, cellulose diacetate and the like, polycarbonates, etc., andnon-magnetic metals, for example, copper, aluminum, zinc, etc. Such anon-magnetic support can have a thickness of about 3 to 100 microns,preferably 5 to 50 microns in the form of a film or sheet.

The above described magnetic powder, binder, dispersing agent,lubricant, abrasive agent, antistatic agent and solvent are well blendedor kneaded to prepare a coating composition. For kneading, the magneticpowder and other components are charged in a kneading machinesimultaneously or separately. For example, a magnetic powder is added toa solvent containing a dispersing agent, kneaded for a predeterminedperiod of time, then mixed with other components and kneadedsufficiently to prepare a magnetic coating composition. Various kneadingmachines are used for the kneading and dispersing, for example, two rollmills, three roll mills, ball mills, pebble mills, trommel mills, sandgrinders, Szegvari attriters, high speed impeller dispersing machines,high speed stone mills, high speed impact mills, kneaders, high speedmixers, homogenizers, ultrasonic dispersing machines, etc. The kneadingand dispersing techniques are described in T. C. Patton, "Paint Flow andPigment Dispersion", published by John Wiley & Sons (1964) and U.S. Pat.Nos. 2,581,414 and 2,855,156.

The foregoing magnetic recording layer can be coated on a support usingcoating methods such as air doctor coating, blade coating, air knifecoating, squeeze coating, dip coating, reverse roll coating, transferroll coating, gravure coating, kiss coating, cast coating, spray coatingand the like, and other coating methods can also be used. These methodsare described in "Coating Kogaku (Coating Engineering)", page 253 to277, published by Asakura Shoten, Tokyo (Mar. 20, 1971).

Typical organic solvents which can be used in the coating includeketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone,cyclohexanone and the like; alcohols such as methanol, ethanol,propanol, butanol and the like; esters such as methyl acetate, ethylacetate, butyl acetate, ethyl lactate, glycol monoethyl ether acetateand the like; ethers and glycol ethers such as diethyl ether, glycolmonoethyl ether, glycol dimethyl ether, dioxane and the like; aromatichydrocarbons such as benzene, toluene, xylene and the like; chlorinatedhydrocarbons such as methylene chloride, ethylene chloride, carbontetrachloride, chloroform, ethylene chlorohydrin, dichlorobenzene andthe like; etc.

The thickness of the magnetic recording layer is generally 0.1 to 10 μm,preferably 0.2 to 5 μm.

Details of other ferromagnetic powders, additives and organic solvent,and dispersing methods and coating methods are as described in JapanesePatent Application (OPI) Nos. 108804/1977, 21805/1979 and 46011/1979.

The present invention will be explained in detail with reference to thefollowing examples. It will be obvious to one skilled in the art thatvarious changes and modifications can be made in the components, ratios,operational order and the like without departing from the spirit of thepresent invention. Therefore, the present invention should not beconstrued as being limited to the following examples. All parts usedherein are to be taken as those by weight unless otherwise indicated.

EXAMPLE 1

    ______________________________________                                        Fe.sub.2 O.sub.3 Powder (Hc 400 Oe, acicular ratio                                                    100 parts                                             10/1, mean grain length 0.4 μm)                                            Vinyl Chloride/Ethylene/Vinyl Acetate/                                                                shown in Table 1                                      Maleic Anhydride Copolymer (polymeriza-                                       tion degree 800, composition as shown in                                      Table 1)                                                                      Polyester Polyurethane (molecular weight                                                              shown in Table 1                                      130,000)                                                                      Oleic Acid               2 parts                                              Methyl Ethyl Ketone     150 parts                                             Cyclohexanone           100 parts                                             ______________________________________                                    

The above described composition was charged in a ball mill, kneaded anddispersed for 48 hours and then filtered through a filter having a meanpore diameter of 7 μm to obtain a coating composition for a magneticlayer.

The thus resulting coating composition was coated onto a polyethyleneterephthalate film with a thickness of 7 μm to give a thickness of 5 μmon dry basis by means of reverse rolls, subjected, while still notdried, to an orientation treatment in a magnetic field using anelectromagnet of 1000 gauss, dried, subjected to a supercalender rolltreatment to smoothen the magnetic layer and then slit to obtain anaudio cassette tape (Philips type compact cassette) with a width of 3.81mm. The properties of the resulting tapes (Sample Nos. 1-5) are shown inTable 1.

COMPARATIVE EXAMPLE 1

The procedure of Example 1 was repeated except using a vinylchloride/vinyl acetate/maleic anhydride copolymer (86/13/1% by weight,degree of polymerization about 400) of the prior art instead of thevinyl chloride/ethylene/vinyl acetate/maleic anhydride copolymer ofExample 1 to obtain a cassette tape. The properties of the resultingtapes (Comparative Sample Nos. 1-3, represented by "C-No.") are shown inTable 1.

                                      TABLE 1                                     __________________________________________________________________________            Composition of                                                                Copolymer    Amount                                                                             Amount                                                      (wt %)       of Copo-                                                                           of   MOL SOL                                                VC E  VAC MA lymer                                                                              PU   (dB)                                                                              (dB)                                                                              TRP                                                                              C TS                                __________________________________________________________________________    Example No. 1                                                                 Sample No.                                                                    1       89 5   5  1  20   --   0.5 0.6 A  A A                                 2       88 5   5  2  20   --   0.2 0.8 A  A A                                 3       79 10 10  1  20   --   0.9 1.7 A  A A                                 4       84 5  10  1  20   --   0.7 1.2 A  A A                                 5       95 2   2  1  18   2    1.1 2.1 A  A A                                 Comparative                                                                   Example No. 1                                                                 C-1     86 -- 13  1  20   --   -0.7                                                                              -2.3                                                                              C  B A                                 C-2     86 -- 13  1  18   2    -0.4                                                                              -1.1                                                                              B  B B                                 C-3     86 -- 13  1  16   4    0.0 0.0 A  B C                                 __________________________________________________________________________

EXAMPLE 2

    ______________________________________                                        Co-doped Berthollide Iron Oxide                                                                       100 parts                                             (Co 3.0 atomic %, FeO.sub.1.4                                                 Hc 660 Oe, acicular ratio 10/1                                                mean grain length 0.4 μm)                                                  Vinyl Chloride/Ethylene/Vinyl Acetate/                                                                shown in Table 2                                      Maleic Anhydride Copolymer (polymeriza-                                       tion degree about 800, composition shown                                      in Table 2)                                                                   Oleic Acid               2 parts                                              Butyl Stearate           1 part                                               α-Alumina (mean grain diameter 0.5 μm)                                                        3 parts                                              Methyl Ethyl Ketone     150 parts                                             Cyclohexanone           100 parts                                             ______________________________________                                    

The above described composition was kneaded and dispersed for 3 hours bymeans of a batchwise sand grinder, mixed with 6.7 parts (solid content5.0 parts) of a polyisocyanate compound (Colonate L-75 made by NipponPolyurethane KK), further kneaded and dispersed for 0.5 hour and thenfiltered through a filter having a mean pore diameter of 1 μm to obtaina coating composition for a magnetic layer.

The thus resulting coating composition was coated onto a polyethyleneterephthalate film with a thickness of 14 μm to give a thickness of 5 μmon dry basis by means of reverse rolls, subjected, while still notdried, to an orientation treatment in a magnetic field using anelectromagnet of 1500 gauss, dried, subjected to a supercalender rolltreatment to smoothen the magnetic layer and then slit to obtain a videocassette tape (VHS type video cassette) with a width of 1/2 inch. Theproperties of the resulting video tapes (Sample Nos. 6-10) are shown inTable 2.

COMPARATIVE EXAMPLE 2

The procedure of Example 2 was repeated except using a vinylchloride/vinyl acetate/maleic anhydride copolymer (86/13/1% by weight,degree of polymerization about 400) of the prior art instead of thevinyl chloride/ethylene/vinyl acetate/maleic anhydride copolymer ofExample 2 to obtain a video cassette tape. The properties of theresulting tapes (Comparative Sample Nos. 4-5) are shown in Table 2.

EXAMPLE 3

The procedure of Example 2 was repeated except using an Fe-Zn alloy finepowder (Fe:Zn=90:10, Hc 1300 Oe, acicular ratio 10/1, mean grain length0.3 μm) instead of the Co-doped Berthollide iron oxide of Example 2 toobtain a video tape. The properties of the resulting video tape (SampleNo. 11) are shown in Table 2.

COMPARATIVE EXAMPLE 3

The procedure of Comparative Example 2 was repeated except using anFe-Zn alloy (Fe:Zn=90:10, Hc 1300 Oe, acicular ratio 10/1, mean grainlength 0.4 μm) instead of the Co-doped Berthollide iron oxide ofComparative Example 2 to obtain a video tape. The properties of theresulting video tape (Comparative Sample No. 6) are shown in Table 2.

                                      TABLE 2                                     __________________________________________________________________________        Composition of     Amount of                                              Sample                                                                            Copolymer (wt %)                                                                           Amount of                                                                           Polyiso-                                                                            Amount                                                                             Video Output                                                                         S/N Ratio                                                                           Video Running                                                                         Still Life             No. VC E  VAC MA Copolymer                                                                           cyanate                                                                             of PU                                                                              (dB)   (dB)  Property                                                                              (min)                  __________________________________________________________________________    6   89 5  5   1  20    5     --   0.7    0.6   A       100                    7   88 5  5   2  20    5     --   0.9    0.7   A       more than 120          8   79 10 10  1  20    5     --   1.5    1.3   A       80                     9   84 5  10  1  20    5     --   1.3    1.0   A       90                     10  95 2  2   1  15    5     5    1.9    1.5   B       more than 120          C-4 86 -- 13  1  20    5     --   -0.6   -1.0  B       40                     C-5 86 -- 13  1  15    5     5    0.0    0.0   B       60                     11  88 5  5   2  20    5     --   12.3   10.8  B       30                     C-6 86 -- 13  1  15    5     5    11.4   9.5   C       10                     __________________________________________________________________________

The abbreviations and measuring methods shown in Table 1 and Table 2 areillustrated below:

Abbreviation

VC: Vinyl Chloride

E: Ethylene

VAC: Vinyl Acetate

MA: Maleic Anhydride

PU: Polyester Polyurethane

TRP: Tape Running Property

C: Contamination

TS: Tape Squeal

Measuring Method

(a) MOL 315

Maximum output level at 315 Hz when that of Comparative Sample No. C-3is 0 dB (harmonic distortion factor 3%); measured by using a meter of582 type made by Nakamichi KK.

(b) SOL 10K

Saturation output level at 10 KHz when that of Comparative Sample No.C-3 is 0 dB.

(c) Tape Running Property

Running tests are carried out at 25° C. and 50% RH, and at 40° C. and80% RH using 100 commercially available cassette decks to assess thewinding state:

A . . . no winding disorder

B . . . winding disorders found in 1-5 decks

C . . . winding disorders found in 5-10 decks

D . . . winding disorders found in 11 or more decks

(d) Contamination

After the assessment of the running property, contamination of a head ina deck is assessed as follows:

A . . . no or little contamination (hardly found)

B . . . some contamination to such an extent that does not matter

C . . . considerable contamination

(e) Tape Squeal

Tape squeal is assessed in the test of the running property:

A . . . no tape squeal

B . . . squeal occurring temporarily in 1-2 windings

C . . . squeal occurring temporarily in 3-5 windings

D . . . temporary squeal occurring in 5 or more windings and constantsqueal occurring in 1-2 windings

(f) Video Output

Output at 4 MHz when that of Comparative Sample No. C-5 is 0 dB (meter:NV-6600 made by Matsushita Denki KK)

(g) S/N Ratio

S/N ratio at 10 KHz to 4 MHz subjected to visibility correction whenthat of Comparative Sample No. C-5 is 0 dB (meter: NV-6600 made byMatsushita Denki KK)

(h) Video Running Property

Running tests are carried out at 25° C. and 50% RH, and at 40° C. and80% RH using 50 commercially available video cassette decks (VHS type)to assess the jitter and skew:

A . . . no jitter, nor skew

B . . . some jitter or skew to such an extent that does not matter

C . . . jitter or skew occurring often

(i) Still Life

Period of time (min) until an image is markedly degraded whenreproduction is carried out at Still Mode, measured using a meter ofNV-6600 made by Matsushita Denki KK.

The following will be apparent from these Examples and ComparativeExamples:

As is evident from Table 1, Audio Cassette Tape Sample Nos. 1-5 usingthe vinyl chloride/ethylene/vinyl acetate/maleic anhydride copolymersaccording to the present invention each have higher MOL and SOL, i.e.more excellent electromagnetic property than Audio Cassette TapeComparative Sample Nos. 1-3 using the vinyl chloride/vinylacetate/maleic anhydride copolymer of the prior art. In addition, theuse of the vinyl chloride/ethylene/vinyl acetate/maleic anhydridecopolymers according to the present invention results in no problem asto the tape running property, contamination and tape squeal, while theuse of the vinyl chloride/vinyl acetate/maleic anhydride copolymer ofthe prior art results in lowering of the tape running property whenusing jointly no polyurethanes, and results in tendency of tapesquealing and deterioration of the durability due to the inferiorhumidity resistance when using jointly polyurethanes.

As is evident from Table 2, Video Tape Sample Nos. 6-10 using the vinylchloride/ethylene/vinyl acetate/maleic anhydride copolymers according tothe present invention each have a higher video output and S/N ratio,i.e. more excellent electromagnetic property, a better video runningproperty, longer still life and more excellent running durability thanVideo Tape Comparative Sample Nos. C-4 and C-5.

As evident from comparison of Sample No. 11 with Comparative Sample No.C-6, the vinyl chloride/ethylene/vinyl acetate/maleic anhydridecopolymer of the present invention is also superior to the vinylchloride/vinyl acetate/maleic anhydride copolymer of the prior art evenwhen using alloy fine powders.

It will clearly be understood from the foregoing results that a magneticrecording medium, which is more excellent in electromagnetic property,tape running property and durability than those of the prior art, can beobtained by the use of the vinyl chloride/ethylene/vinyl acetate/maleicanhydride copolymers according to the present invention.

What is claimed is:
 1. A magnetic recording medium comprising anon-magnetic support and a magnetic layer provided thereon, said layercomprising a ferromagnetic powder dispersed in a binder consistingessentially of a copolymer of 50 to 95 percent by weight of vinylchloride, 0.5 to 20 percent by weight of ethylene, 0.5 to 20 percent byweight of vinyl acetate, and 0.5 to 5 percent by weight of maleicanhydride, wherein the copolymer has a degree of polymerization of 200to
 2000. 2. The magnetic recording medium of claim 1, wherein a part orall of the maleic anhydride is replaced by at least one other carboxylgroup-containing monomer selected from the group consisting of acrylicacid and methacrylic acid.
 3. The magnetic recording medium of claim 1,wherein the binder is contained in a proportion of 10 to 50% by weightto the ferromagnetic powder.
 4. The magnetic recording medium of claim1, wherein the ferromagnetic powder is selected from the groupconsisting of powders of γ-Fe₂ O₃, Co-doped γ-Fe₂ O₃, Fe₃ O₄, Co-dopedFe₃ O₄, Berthollide compounds of γ-Fe₂ O₃ and Fe₃ O₄, Co-dopedBerthollide compounds of γ-Fe₂ O₃ and Fe₃ O₄, CrO₂, Co-Ni-P alloys,Co-Ni-Fe alloys, Co-Ni-Fe-B alloys, Fe-Ni-Zn alloys, Fe-Mn-Zn alloys,Fe-Co-Ni-P alloys and Ni-Co alloys.
 5. The magnetic recording medium ofclaim 1, wherein the ferromagnetic fine powder has a mean grain size of0.2 to 1 μm in length with a length to width ratio of 1:1 to 20:1. 6.The magnetic recording medium of claim 1, wherein the binder furthercontains at least one other resin miscible with the vinylchloride/ethylene/vinyl acetate/maleic anhydride copolymer, wherein saidother resin is selected from the group consisting of cellulose resins,vinylidene chloride/acrylonitrile copolymers, vinyl chloride/vinylacetate copolymers, vinyl chloride/vinyl acetate/vinyl alcoholcopolymers, vinyl chloride/vinyl acetate/maleic anhydride copolymers,polyurethane resins, and polyvinyl butyral resins.
 7. The magneticrecording medium of claim 6, wherein at least one of other resins iscontained in a proportion of at most 40% by weight.
 8. The magneticrecording medium of claim 1, wherein the binder contains further atleast one polyisocyanate hardeners.
 9. The magnetic recording medium ofclaim 8, wherein the polyisocyanate hardeners are reaction products of 3moles of a diisocyanate and 1 mole of trimethylolpropane, biuret adductcompounds of 3 moles of hexamethylene diisocyanate, isocyanurate adductcompounds of 3 moles of toluylene diisocyanate and 2 moles ofhexamethylene diisocyanate, polymer compounds of diphenylmethanediisocyanate and isophorone diisocyanate.
 10. The magnetic recordingmedium of claim 8, wherein the polyisocyanate hardener is contained inthe binder in a proportion of at most 40% by weight.